According to this article it isn't due to Google Voice, but due to a flood of iPhone users bypassing their SMS plan limitations by exploiting the Google Talk SMS capability.
Infinite SMS is a third party app that has been using Google technology to provide free SMS for users, while we were paying for the cost of the text messages. While Google is supportive of third party apps, we've decided we can't support this particular usage of our system at this time. SMS chat is still just an experiment in the early testing stages in Gmail Labs. We're blocking all external XMPP clients from sending SMS; we're not singling out Inner Fence.